Update 2: O'Shaughnessy Dam releases

Cool weather continues to preserve the snowpack and moderate inflows to Hetch Hetchy Reservoir, which has been allowed to rise above 300,000 acre-feet to prepare for UTREP high flow releases this week.   Releases began yesterday (May 23rd) as part of a slightly modified schedule.

In order to confirm Poopenaut Valley wetland inundation flow thresholds, bench releases were made today at 3,300 cfs and 3,700 cfs. These will be followed by releases at 4,800 cfs (May 25 @ 08:00) and 5,700 cfs (May 25 @ 15:00). The peak release of 8,500 cfs will begin on May 26 @ 08:00 and begin to ramp down on May 28 @ 08:00. Following the peak, releases will be ramped down to 5,700 cfs to support Poopenaut Valley inundation.  These releases are being monitored by equipment in Poopenaut Valley and by NPS staff as part of Yosemite National Park's Looking Downstream Project.

Releases are currently 3,700 cfs from O’Shaughnessy Dam, 700 cfs from Kirkwood powerhouse, 1,900 cfs from Cherry Creek, 900 from the Middle and South Fork Tuolumne, and about 200 cfs of accretion flow. Dreamflows is a few hours behind recent changes (due to travel time) and is at 6,400 cfs and on its way to 7,400 cfs.  See realtime flow links at right for current flow information.

Flow estimates (plus or minus 500 cfs) for Lumsden are:

May 25: 7,400 cfs in the morning, rising through the day to 9,400 cfs around 6 pm
May 26: 9,400 cfs in the morning, rising to 12,200 cfs around 1 pm
May 27: 12,200 cfs all day
May 28: 12,200 cfs in the morning, decreasing to 10,100 cfs around 3 pm.
May 29: 10,100 cfs in the morning, decreasing to 9,400 cfs around 3 pm

Releases from O'Shaughnessy Dam will hold constant from May 30 through at least June 3, and the approximately 9,400 cfs at Lumsden will rise or fall based on the tributary contributions and snowmelt. If warmer weather follows the cold Memorial Day weekend, the ~9,400 cfs will continue to June 8.

Update: O'Shaughnessy Dam high flow releases

Due to cool temperatures and some highly variable weather, inflows to Hetch Hetchy Reservoir have dropped significantly.  In the last 4 days at 8,500 ft in the Tuolumne watershed, conditions have gone from 65F daytime highs at 8,500 feet to 3” of new snow at 8,500 ft.  Cool conditions are expected to persist through May 17th.  The low inflows mean that Hetch Hetchy operators will not have adequate levels of water storage in the reservoir to implement the "test drive" of ecological high flow releases as planned next week.  Instead, the planned high flow test drive will be delayed by a week; Hetch Hetchy operators plan to begin ramping on Sunday, May 22.

Here's the current plan for the releases at O'Shaughnessy Dam (OSD) and resulting flows at Lumsden:

• May 12 - 22:  OSD release of 1,400 cfs.  Lumsden flows will be approximately 5200 when combined with Cherry, Kirkwood powerhouse, and Middle and South Forks flows;
• May 23 -24:  OSD release of 3,300 cfs.  Lumsden flow will be approximately 7100 cfs;
• May 25:  OSD release of 4,800 cfs.  8,800 cfs at Lumsden;
• May 26:  OSD release of 5,700 cfs.  9700 cfs at Lumsden;
• May 27 – 29:  OSD release of 8,500 cfs.  12,500 cfs at Lumsden;

The length of the 8,500 cfs high flow release will depend on weather and inflows.  Wetland inundation bench flows will be released after the peak, however the timing will again depend on weather.  High flows at Lumsden (>9,000 cfs) are likely to continue – assuming normal spring/warming weather – through at least June 8.  We'll provide updates here as the snowmelt season unfolds.

High flow releases from O'Shaughnessy Dam

This has been a very wet winter with recent snow survey results indicating the snowpack in the Upper Tuolumne watershed at 184% of average May 1st snow conditions.  Currently, the snow line is near 5600 feet and has melted on south and west slopes up to approximately 6400 ft; above 6400 feet the snow coverage is 100%.  Warm temperatures over the last week have been melting this low elevation snow and generating streamflows in the watershed. Over all, SFPUC hydrologists believe over 500,000 acre-feet of excess runoff ("spill") is likely to occur over the next 2-3 months at O'Shaughnessy Dam.

As discussed at the April 8 stakeholder meeting, draft UTREP flow recommendations for O'Shaughnessy Dam include high flow releases during the snowmelt season to meet sediment transport and Poopenaut Valley wetland inundation objectives.  This year, the SFPUC and NPS will take those recommendations for a "test drive" to help identify any potential operational issues.  Beginning May 15th, operators at O'Shaughnessy Dam will begin high flow releases, starting with test wetland inundation "benches" at 3300, 3700, 4800, and 5700 cfs.

High flow release from O'Shaughnessy Dam in 2009. (W. Sears photo)

These releases will be monitored by equipment in the Poopenaut Valley and NPS staff will take photos to confirm inundation of wetland surfaces.  A peak release for sediment transport of 8500 cfs is targeted for May 19th and 20th, followed by a 5700 cfs bench or a continued 8500 cfs release if it is necessary to control Hetch Hetchy reservoir elevations. Although variable weather conditions could require changes to this plan, the "test drive" will provide us with a valuable opportunity to evaluate the operational procedures needed to implement a new set of flow recommendations at O'Shaughnessy Dam.